Internal combustion engines operating on a six-stroke cycle are generally known in the art. In a six-stroke cycle, a piston reciprocally disposed in a cylinder moves through an intake stroke from a top dead center (TDC) position to a bottom dead center (BDC) position to admit air, an air/fuel mixture, and/or an air/exhaust gas mixture into the cylinder. During a compression stroke, the piston moves towards the TDC position to compress the air or the air/fuel/exhaust gas mixture. During this process, an initial or additional fuel charge may be introduced to the cylinder by an injector. Ignition of the compressed mixture increases the pressure in the cylinder and forces the piston towards the BDC position during a first power stroke. In accordance with the six-stroke cycle, the piston performs a second compression stroke in which it recompresses the combustion products remaining in the cylinder after the first combustion or power stroke. During this recompression, any exhaust valves associated with the cylinder remain generally closed to assist cylinder recompression. Optionally, a second fuel charge may be introduced into the cylinder during recompression to assist igniting the residual combustion products and produce a second power stroke. Following the second power stroke, the cylinder undergoes an exhaust stroke with the exhaust valve or valves open to substantially evacuate combustion products from the cylinder. One example of an internal combustion engine configured to operate on a six-stroke cycle can be found in U.S. Pat. No. 7,418,928. This disclosure relates to a method of operating an engine that includes compressing part of the combustion gas after a first combustion stroke of the piston as well as an additional combustion stroke during a six-stroke cycle of the engine.
Some possible advantages of the six-stroke cycle over the more common four-stroke cycle can include reduced emissions and improved fuel efficiency. For example, the second combustion event and second power stroke can provide for a more complete combustion of soot and/or fuel that may remain in the cylinder after the first combustion event. However, the additional piston strokes and fuel charges may increase the complexity of the internal combustion engine and its operation. The present disclosure is directed to addressing the increased complexity of the engine.